_Artificial Colouring._--The list of colouring matters said to be added to butter includes the vegetable dyes, annato, carotin, fustic, turmeric, marigold, and saffron; the coal-tar colour, Victoria yellow (potassium dinitrocresylate), and Martius yellow (potassium dinitronaphthalate), and the mineral pigment chrome yellow (plumbic chromate). Of the foregoing, annato and carrot colour appear to be most commonly employed. Mr. Edward W. Martin[38] has proposed a method for the isolation of the former which consists in dissolving the butter in carbon disulphide, and shaking the solution with a _dilute_ solution of potassium hydroxide, in which the colouring matter dissolves; it is subsequently identified by further tests. According to Mr. R. W. Moore,[39] the presence of carotin in butter may be detected by first agitating the carbon disulphide solution of the fat with alcohol, which fails to extract this colour. Upon now adding to the mixture a drop of dilute ferric chloride solution, again shaking the liquid and then putting it aside for a short time, the alcoholic solution dissolves the carrot colour, and if no other colouring matter is contained in the butter, leaves the carbon disulphide colourless. The artificially coloured butter may be dissolved in alcohol and tested with the following reagents:-- (_a_) Nitric acid: greenish coloration, _saffron_. (_b_) Sugar solution and hydrochloric acid: red coloration, _saffron_. (_c_) Ammonia: brownish coloration, _turmeric_. (_d_) Silver nitrate: blackish coloration, _marigold_. (_e_) Evaporate the alcoholic solution to dryness and add concentrated sulphuric acid: greenish-blue coloration, annato; blue coloration, _saffron_. (_f_) Hydrochloric acid: decolorisation, with formation of yellow crystalline precipitate, _Victoria or Martius yellow_. (_g_) Separation of a heavy and insoluble yellow powder, _chrome yellow_ (see p. 130). _Microscopic Examination._--The microscopic examination of butter has lately received considerable attention as a means for the detection of the presence of foreign fats. Genuine butter generally exhibits under the microscope a crowded mass of globules of fat, fatty crystals being commonly absent. In oleomargarine a more crystalline structure is observed, with pear-shaped masses of fat and but few globules. While the presence of crystals in a sample may justly be regarded as suspicious, it is by no means a positive evidence of adulteration, since, under certain circumstances, pure butter may present the same indications. In applying the microscopic test, a small portion of the fat is made into a thin layer on the slide, and then protected with a glass cover, applied with rather gentle pressure.